The invention relates to vapor deposition systems and, in particular, to methods and apparatus for forming a metal coating modified by the inclusion of a secondary element on a gas turbine engine component.
Gas turbine engines are used as aircraft or jet engines (e.g., turbofans) and as industrial gas turbine engines for power generation. Gas turbine engines contain components, such as turbine blades, vanes, shrouds, and nozzle guides, formed from metallic superalloy materials, such as a nickel-based superalloy or a cobalt-based superalloy. Superalloy components are exposed to extreme operating conditions in a gas turbine engine. Air flow or gas-washed surfaces may be directly contacted by the hot exhaust gases, which heat these components to high temperatures and expose them to impurity elements originating from the combusted fuel. Consequently, superalloys are susceptible to high temperature oxidation in this harsh environment.
Different protective coatings are used for components in the various stages of a gas turbine engine. The air flow or gas-washed surfaces on superalloy components in high-pressure stages are usually protected against high temperature oxidation by a sacrificial intermetallic coating, such as a diffusion aluminide coating. While the turbine engine is operating, a superficial oxide layer forms that protects the underlying superalloy base metal from high temperature oxidation. Unfortunately, the sacrificial intermetallic layer thins during engine operation and, eventually, must be replaced. Before a new sacrificial intermetallic layer is applied, the thinned layer is stripped from the superalloy base metal. This replacement process is repeated as many times as possible to prolong the service life of the turbine engine component in the gas turbine engine.
One technique for applying the sacrificial intermetallic coating on a turbine engine component is a simple chemical vapor deposition (CVD) process. A cleaned turbine engine component is placed into a reaction chamber containing an activator material and a donor material including at least one metal to be integrated into the alloy forming the intermetallic coating. The reaction chamber is purged of atmospheric gases and evacuated. By elevating the temperature of the reaction chamber, the activator material and donor material are heated to generate vapor phase reactants that cause metal to be transported from the donor material to the turbine engine component. The intermetallic coating formed on the turbine engine component may include intrinsic metal diffused outwardly from the alloy forming the turbine engine component. However, the intermetallic coating must also include at least one extrinsic metal originating from the donor material.
Diffusion aluminide coatings may be modified with platinum to improve their high-temperature oxidation resistance during cyclic oxidation. The improvement is believed to be primarily attributed to the corresponding enhancement of the adherence of the alumina scales in the protective oxide layer to the coated substrate in the presence of platinum. Despite this improvement in properties, replacements are being sought for platinum-modified diffusion aluminides.
Chromide coatings are also potentially capable of serving as protective coatings for superalloy components in the gas turbine engine. In particular, chromide coatings are favored for components in low-pressure stages of the gas turbine engine that are typically exposed in service to intermediate-range temperatures. Similar to aluminides, chromide coatings usually interdiffuse with the base metal in the substrate.
The properties of chromide and diffusion aluminide coatings may be improved by introducing performance enhancing secondary elements into the alloy. For example, modifications of silicon or hafnium are recognized to improve the high-temperature oxidation resistance of chromide and diffusion aluminide coatings. However, a problem that has been observed is an inability to reproducibly control the amount of silicon or hafnium that is introduced as a performance enhancing secondary element into the coating.
Accordingly, there is a need for improved methods and apparatus for forming a modified metal coating on superalloy gas turbine engine components.